The invention relates to the field of electromagnetic coupling applied to the field of exploration and working oil or gas fields in which mutually communicating drill strings are used, constituted by tubular components such as standard drill pipes, which may be heavy weight, and other tubular elements, in particular drill collars in the bottom hole assembly, connected together end-to-end as required by the drilling process
Drilling for oil and the pipeline field are fields in which the transmission of information has become a determining element.
However, certain cutting edge industrial fields such as drilling for oil have operational environments that render data transmission difficult.
As an example, in the context of drilling for oil, measurement means are disposed at the deepest tubes of the drill string. Such measurement devices are used to pick up data pertaining to the drilling environment, especially with a view to directing the drilling.
Bringing that data to the surface is a major problem because the operating environment for such tubes is hostile and renders the use of conventional telecommunication means impossible.
The operational environment in fact poses many problems as regards the supply of the various elements. Furthermore, that environment is also the source of numerous interferences which perturb the signal along the tube string.
Two principal technologies have been developed in response.
The first of those technologies consists of sending the data through the mud moving in the string via sound waves. That method has proved to be highly insufficient in terms of rate, as it can only offer rates of the order of one to a few bits per second.
The second technology, which is still being developed, uses cabled tubular connections coupled to techniques for coupling by magnetic induction. Thus, a coupling element is disposed at each end of each tube, and a wire connects the coupling elements of each tube. It is then possible to transmit the signal from tube to tube along the string, the coupling elements at the end of two successive tubes ensuring transmission between those two tubes.
That technology can be used to increase the rates to a few kilobits per second. However, that increase in rate is at the expense of limited reliability. Further, the losses at each pair of coupling elements of consecutive tubes are high, which means that a lot of supply repeaters have to be included in the string in order to amplify the signal level. Such repeaters are expensive, difficult to maintain and are difficult to incorporate into the design of the drill stem.
In the pipeline field, the operating environment is also very aggressive, and of little use to wireless communications. Thus, it is still necessary to provide cabled connections.
In order to connect two cabled portions of a unit, a coupler then becomes necessary. However, couplers with contacts suffer from many disadvantages in an aggressive environment. In response to this problem, contactless couplers have been developed. However, such couplers cannot be used to obtain good performances in transmission.
In the prior art, the documents GB-2445207, US-2004-0094303, U.S. Pat. No. 6,392,317 and US-2010-0052941 disclose various solutions for coupling drill strings together.
Currently, no coupler, with or without contact, is satisfactory for the transmission of information over long distances in a hostile environment.